Electric protective system



June 8, 1937. c. L. LOUDON ELECTRIC PROTECTIVE SYSTEM Filed Oct. 12, 1953 lNVENTO/P C. L. LOUDON Br V g ATTORNEY Patented June 8, 1937 UNETED STATES PATENT l l 'ltlE Holmes Electric Protective Company,

New

York, N. Y., a corporation of New York.

Application October 12, 1933, Serial No. 693,252

12 Claims.

This invention relates to electric protective systems and more particularly to burglar alarms of the type in which the approach of an intruder at a place to be protected is registered by an alarm at the remote station of an officer or guard.

An object of the invention is to provide a protective device which shall be very sensitive to the approach of intruders.

Another object of the invention is to insure that a guard or a watchman at a remote point will receive an alarm indication whenever an intruder enters an area or building to be protected.

A further object of the invention is to transmit-to a remote station an indication that a detecting device is not in working order whenever for any reason that device becomes inoperative as in the of failure of power or of some of its elements, such as the vacuum tube or upon destruction of or tampering with any of its cs- 20 sential elements.

A further object of the invention is to render a detecting device for protecting systems stable and unafiected by variations in temperature.

Another object of the invention is to render 25 a protective device immune from casual electric disturbances which are not associated with the app-roach of an intruder.

According to the invention, a vacuum tube oscillator circuit which is so constructed as to 39 have an extremely steep space current characteristic is set to operate at an intermediate point on that characteristic. An antenna or detecting conductor arranged in the vicinity of the vault or area to be protected is connected to the circuit so that upon approach of an intruder the capacity of the antenna varies changing the natural frequency of a tuned circuit which constitutes the output circuit of the oscillator and, in consequence thereof, causing the space our- 46 rent to change. A series marginal relay of the overload type connected in the space current circuit is thereupon operated to produce an alarm signal or indication at a watchmans station. Should the antenna become detached or should the tube of the oscillator or the power supply fail, an opposite change in space current will occur to give a difierent type of signal at the watchmans station.

The invention. itself both as to the details of its organization and its mode of operation will be best understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 illustrates schematically a circuit diagram of a protective system in accordance with the invention.

Fig. 2 is a graph illustrating the relation between the space current in the oscillator and the natural frequency of the tuned circuit, and

Figs. 3, 4 and 5 illustrate modifications of the antenna of the circuit of Fig. 1.

Referring to Fig. 1 there is indicated a safe I which typifies any object or area to be protected. Positioned adjacent to and preferably above the safe is an antenna or detecting conductor element 2. Connected to the antenna is a vacuum tube oscillator detecting apparatus which is enclosed in a box or container 3 preferably of shielding material as, for example, iron. The oscillator is connected by two-wire circuit 4 to indicating relays 5 and 6 at the remote watchmans station 1.

The oscillator, as illustrated, comprises a vacuum tube 8 of the five-element or pentode type. The grid circuit of the oscillator connected between the control grid 9 and the indirectly heated cathode ID, includes a piezo electric crystal ll shunted by a grid leak resistor 12. The crystal may be of quartz or tourmaline and should have a high natural resonance frequency corresponding to the frequency at which the oscillator is to operate. The plate circuit of the oscillator connected between the anode l3 and the cathode Ill includes a tuned loop consisting of inductance l4 and a capacity element l5 tuned to a frequency substantially equal to but slightly lower than the natural frequency of the piezo electric crystal H. The plate circuit for high frequency oscillations is completed through a large blocking capacity I6 which effectively insulates the cathode from the high direct current potential of the space current source H. The space current path passes from the anode l3 by way of the inductance l4, choke coil 31 which excludes high frequency current from the space current source, outgoing conductor of circuit 4, relays 5 and 6 in series, the return conductor of circuit 4 and the space current source ll to the cathode l0. Shunted across the space current source I! is a potentiometer 18 from which a variable tap connection l9 leads by way of choke coil 20 to the screen grid 2|. The screen grid is, accordingly, polarized to a suitable positive potential somewhat lower than that of the anode l3. The screen grid 2! is shielded from alternating current potentials which might exist in the potentiometer l8 by means of the choke coil 20 and, also, by the large shunting or draining capacity element 22 which maintains the shield grid at substantially the cathode potential with respect to alternating current, While serving eifectively to block the direct current polarizing potential impressed on the screen grid from reaching the cathode ID. The pentode tube 8 also contains a suppressor grid 23 positioned between the anode l3 and the shield grid 2| to prevent secondary electron emission from the anode Hi from reaching and affecting the shield grid 2!. The suppressor grid is connected in the well-known fashion directly to the cathode Ill. The tube 8 also contains a cathode heating element 24, which, in series with the resistance 25, is connected across space current source IT. The cathode IE! is directly connected to ground at 26 and the shielding container is likewise connected to ground Fig. 2 illustrates the space current character istic of the circuit of Fig. 1 as the resonance frequency of the tuned circuit [4, i5 is varied by changing the capacitance of variable capacity element I5. Beginning with the point P0 at which the space current is nearly at maximum value, the space current decreases very rapidly with increase in resonance frequency of the tuned circuit reaching a minimum magnitude v at point P2 which corresponds in its frequency F2 with the natural frequency of the crystal Ii. As the natural frequency of the tuned circuit continues to increase from F2 to F3, the space current rises but the curve throughout this range is very much less steep. The loop circuit l4, I5 is preferably tuned to have its resonance frequency at point P1 at approximately the middle of the steep portion of the curve. Accordingly, slight changes in the resonance frequency of the loop 14, I5 will be attended with large changes in the space current. The capacitance of the antenna or detecting conductor 2 to earth may be regarded as that of a condenser, the upper terminal of which is connected through variable capacity 28 to the upper terminal of capacity 15 and the lower or ground terminal of which connected by way of ground 26 and blocking capacity I6 to the lower terminal of capacity element l5. Since capacities l6 and 28 are relatively large the effective capacity of this shunt path will be very largely determined by the capacity to ground of the conductor 2. It follows that any change in the capacity of conductor 2 such as occurs upon approach of an intruder will directly affect the natural frequency of tuned loop l4, l5 and will, correspondingly, affect the space current of the oscillator.

In order to take advantage of the rapid change in space current with change in the natural frequency of the tuned output circuit, the marginal relay 5 in the space current circuit is adjusted to remain unoperated at the normal space current of point P1 but to operate if the space current increases to any considerable extent. Accordingly, should an intruder approach the safe I the capacity from antenna 2 to ground will increase thus increasing the capacity in shunt to element 15. The natural frequency of loop l4, l5 accordingly decreases toward F0 and the space current rises. Marginal relay 5 accord-- ingly responds and closes local circuits which respectively include a signal lamp 38 and a buzzer 39.

In order to determine the resultant effect of a change in the antenna capacitance upon the tuning of the loop l4, 15, the capacity element 28 is made variable. It is, therefore, possible to make the device sensitive enough to changes in capacity of the antenna 2 to assure an indication upon approach of an intruder or any object but to prevent response to slight casual or fortuitous changes in that capacity which may occur.

It is, of course, possible that something may occur to cause the natural frequency of the tuned circuit to increase from frequency F1 past F2 toward frequency F3. This may occur should the antenna 2 become accidentally detached from the apparatus. Under these circumstances, the space current would fall to the minimum at point P2 and would then rise until it reached magnitude sufficient to operate the relay 5. In order to distinguish an event of this sort from the approach of a marauder, the marginal relay 6 included in the space current circuit is so adjusted as to remain energized at all space currents above and also at those slightly below the magnitude of that at point P1. However, should the space current decrease materially below that indicated by P1 relay 6 will release closing local circuits to operate a signal lamp 40 and a buzzer d I. It will, therefore, be apparent at the Watchinans station from the indications given by rclay ii that the oscillator is out of adjustment and requires attention. A similar indication will be given should the space current be diminished or interrupted because of failure of a tube or of the space current power supply. Accordingly, it possible i or the attendant at the watchmans station 7 to distinguish between alarm signals indicating the approach of intruders and those given when the apparatus for any reason departs from its proper operating condition. This enthc watchman to know at all times that apparatus is in proper operating condition when he receives no indication. The piezo electric crystal serves as a stable tuned device of 11.. h selectivity, controlling the frequency of oscillations. As long the natural frequency of circuit M, 15 remains within the narrow frequency range adjacent the excitation frequency of the crystal I l, the oscillations are at high amplitude and the grid leak. resistor circuit by its Well-known action develops a high negative voltage on the control grid 9. As long as grid 9 remains at a relatively high negative potential the space current remains low. Whenever the tuning of the circuit M, 5 departs materially from the frequency of the crystal excitation, the impedance of the circuit M, l5 to the oscillatory current falls off and the effective voltage of the oscillation frequency across the tuned circuit diminishes. In turn, the emf. of the oscillation frequency applied to the grid circuit decreases. the control grid potential becomes less negative, and the space current rises. Accordingly, the crystal H is a very important element of the circuit because oi its effect in making the space current characteristic extremely steep. This makes the device so sensitive that a relatively simple single tube circuit is much more effective than previous complicated and costly circuits in-- volving several tubes.

In an rtus which proved very suc- CA tube of the No. 88 type was used. & ccndcnser had an approximate cum value of .000035 mfd. Capacity 28 had us of 38802 5 mfd. Blocking con- Grid were each of .006 mfd. ranged from 1 to 5 megohms. Poten ornoter resistance iii was of 25,000 ohms and source H of volts. The enclosing containsteel of an iron box of material. Hy, the circuit very readily adaptable frequency less.

to use with ordinary lighting circuits since but a single supply emf. is required.

In operation with the particular apparatus the magnitudes of the circuit elements of which have been given the minimum space current at point P2 approximated 2 milliamperes, and the maximum about 10 milliamperes. The working point P1 was set at about 4 milliamperes. The overload relay was set to operate at currents in excess of 5 milliamperes and the underload relay to release when the current was less than 3 mil1iamperes.

As antenna or detecting conductor element 2, a No. 22 gauge wire has proved quite satisfactory but obviously a wide range of conducting devices may be used depending upon the circumstances.

The crystal frequency may likewise be chosen at any desired value. In the apparatus for which magnitudes have been given frequencies of 2700 ho. to 3600 kc. have been found convenient. These two frequencies correspond approximately to X cut quartz crystal thicknesses of .033 and .022" respectively.

The pentode tube was used in lieu of the ordinary three-element tube because on account of its higher amplifying power it permitted the grid circuit to operate with oscillations of lower amplitude and accordingly developed less heat in the crystal thus tending to affect the crystal It is, of course, possible to use tubes having the suppressor grid 2| polarized with respect to the cathode or even to use a three-element or other entirely different type of tube, if desired.

It is well known that the natural frequency of piezo electric crystals is to some extent dependent upon atrnospheric conditions. Under some circumstances it may be desirable to use crystals of special design which have substantially zero temperature coeificient characteristic or whose temperature coefficient is very low. Where the temperature variations to which the apparatus is subject are severe as in the case of shops and warehouses exposed to atmospheric temperatures, it may be desirable to maintain the crystal ii at an approximately constant temperature in order to stabilize its frequency. Th s may be accomplished by enclosing the crystal in a thermally insulated container 32. The temperature of the atmosphere within the container is regulated by means of a heating resistance 33 in series with a thermostatic circuit closer and a fixed resistor 35. When the temperature falls below a definite point preferably somewhat above the highest normal atmospheric temperature, the thermostatic circuit closer 3 3 closes permitting plate current source ll to supply heating energy to the heating element When the temperature within the container passes the predetermined point the circuit closer 3% opens the circuit of the heater The container 32 may in some instances be dispensed with, the shielding container 3 serving in its stead as a heat confining enclosure.

.iig. 3 illustrates a modification of the antenna conductor in which the conductor is connected directly to the outer shell of the safe or vault to be protected, which thus becomes a fundamental. part of the antenna system. The safe is mounted upon insulating castors or supports Fig. 4 illustrates a modification of antenna assemblage for protecting a wall in which the antenna comprises a continuous conductor or ribbon to be placed within or adjacent the wall to be protected.

Fig. 5 shows another modification in which the antenna comprises a plurality of conductors connected in a multiple group and placed within or adjacent to a wall in the same manner as the conductor of Fig. 4.

Although the watchmans station 1 is indicated as remote from the detecting apparatus, it will be readily appreciated that in many instances it may be in the same or a neighboring building. Where the separation is relatively great as in a central protective system in which relays 5 and 5 may be one set of a large number of similar devices on a central station alarm board it may be undesirable extend the space current circuit 4 to the remote central station alarm board. In such cases the apparatus indicated at station i may be located adjacent the detecting apparatus 3 and the alarm circuits of devices 38, 39, 4c and M may be extended from that point to the central station board upon which the lamps and buzzers may be mounted.

What is claimed is:

1. In combination, a source of high frequency oscillations, said source comprising an electron discharge device having a cathode, an anode and an impedance control element, a piezoelectrio element and. means connecting said piezo electric element to said cathode and impedance control element, a resonant circuit connected between said cathode and anode and normally tuned to a frequency slightly lower than the natural frequency of said piezoelectric element whereby the oscillator normally oscillates at a point in the frequency zone in which the space current changes most rapidly with change of frequency of the resonant circuit, a capacity member connected to said resonant circuit and adapted to cause the resonance frequency of the resonant circuit to vary upon approach of an object, a translating device, and means controlled by variations in the magnitude of the space current of said electron discharge device for operating said translating device when the capacitance which said capacity member occasions is either abruptly increased or abruptly diminished from its normal magnitude.

2. In combination, an electrical oscillator including an electron discharge device having a cathode, an impedance controlling element, an anode and a shield grid, and input circuit including a piezoelectric element and a grid leak resistance in parallel with the piezoelectric ele ment having its terminals respectively connect-- ed to the cathode and the impedance controlling element, a circuit connecting the anode and cathode and tuned to a frequency slightly lower than the natural resonance frequency of the piezoelectric element in the region in which the space current of the oscillator experiences its most rapid change for slight changes in frequency of the tuned anode-cathode circuit, means for polarizing the shield grid positively with respect to the cathode to increase the output oscillation amplitude relative to the amplitude of the oscillations of the grid circuit, means including a control conductor connected to the tuned circuit for varying the resonance frequency of the tuned circuit, and a space current path connecting said cathode and anode and including a space current source and a device responsive to change in space current whereby a large change in space current occurs if the control conductor capacity changes or if the control conductor becomes detached from said tuned circuit.

3. An electric discharge oscillator comprising an electric discharge device having a cathode, grid and anode, a tuned circuit connected to the anode and cathode, a detecting conductor connected to the tuned circuit to cause the natural frequency of the tuned circuit to vary upon approach of an external object, a piezoelectric element connected to said grid and cathode, the piezoelectric element having a natural frequency approximately equal to but slightly higher than the normal frequency of said circuit, a path including a source of space current connecting the anode and cathode, and a metallic shielding enclosure surrounding all of said oscillator including its piezoelectric element and tuned circuit except the detecting conductor whereby the effeet produced by external conditions on the frequency of the oscillations is restricted to the effect on the detecting conductor whereby variation in the electrostatic environment of the detecting conductor to cause the natural frequency of the tuned circuit to change with re-- spect to the natural. frequency of the piezoelec tric element results in extremely rapid change in space current.

4. An electric discharge oscillator, according to claim 3, characterized in this, that there is provided means for maintaining the temperature of said piezoelectric element approximately constant to prevent variations in its natural frequency.

5. In combination, an electrical oscillator comp1"'"ng a space discharge device, the space disirge device including a cathode, an anode, an iedance controlling grid, a screen grid and a suppressor grid, an input circuit including a piezoelectric element connecting the cathode and impedance controlling grid, a grid leak path of h gh resistance in shunt to the piezoelectric ele ment whereby the grid polarization and the re sultant space current are functions of the amplltude of the oscillations produced, an oscillation circuit connected to the anode and cathode and tuned to a frequency slightly different from the natural frequency of the piezoelectric element in the region in which the space current of the oscillator changes rapidly with change in the resonance frequency of the oscillation circuit, means connectlng the suppressor grid to the cathode and means to polarize said screen grid positively wil respect to the cathode whereby an augmented amplification results that permits operation at higher oscillation amplitude levels in the oscillation circuit with decreased magnitude of inmput circuit oscillations, a control conduct-or connected to the oscillation circuit and adapted to change the resonance frequency of the oscillation c rcuit upon change of the capacitance oi the control conductor, and means responsive to a large change in the space discharge current of said oscillator.

6. An electric system comprising a control con ductor, a space discharge relay, an input circuit and an output circuit each connected to said relay, an electromechanical resonance dev c connected in said input circuit, a selective Cll'Cllit tuned approximately to the resonance frequency of the electromechanical device connected in said output circuit whereby oscillations are produced with an amplitude which decreases as the resonance frequency of the electromechanical device and the tuning frequency of the selective circuit diverge, means for controlling the mag- 7 nit-ude of the space current of the space discharge device in accordance with the magnitude of the oscillations produced, and means connecting the control conductor to the selective circuit whereby upon change of the electrostatic environment of the control conductor the frequency of the selective circuit is altered.

7. In combination, an electric oscillator, comprising a pentode space discharge device having a cathode, an anode, a control grid, a screen grid and a suppressor grid, an input circuit connected to the cathode and control grid of said discharge device including a piezoelectric element, an output circuit including a source of space current connected to the cathode and anode of said space discharge device including a tuned circuit resonant at approximately the same frequency as the piezoelectric element for causing said space discharge device to operate on a very steep portion of its space current frequency characteristic, a connection from the suppressor grid to the cathode, a lead connecting the screen grid to a point in the external anode cathode circuit which is positive with respect to the cathode, a control conductor connected to the tuned circuit, and means responsive to change in the space current of said device in consequence of variations in the effective reactance which the control conductor presents to the tuned circuit upon change of its electrostatic environment.

8. In an electric system, an electrical oscillator comp -ising a space discharge device having a cathode, an anode, and an impedance controlling grid, an input circuit for said oscillator connected to said cathode and grid including a piezoelectric element and a grid leak path in shunt thereto, an output circuit connected to said anode and cathode and including a loop tuned to approximately the natural frequency of the piezoelectric element for making the space current characteristic of said oscillator with respect to the resonance frequency of the tuned loop very steep, a control conductor electrically connected to said tuned loop and adapted upon change of its capacitance or upon disconnection from the loop to cause the resonance frequency of the loop to change, means for keeping the natural frequency of said piezoelectric crystal substantially constant regardless of atmospheric conditions, means for preventing changes in the external. electrostatic environment from affecting the frequency of the oscillation produced by the oscillator except as they affect the control conductor, and means connected in a space current path between the anode and cathode and operating in response to change in the magnitude of the space discharge current of said oscillator.

9. In combination, an electron discharge de vice having a cathode, an anode, an impedance control grid, a screen grid located between the anode and impedance control grid, a suppressor grid located between the anode and the screen grid, an input circuit including a piezoelectric element and a grid leak resistor connected in parallel between the control grid and cathode, a closed loop circuit connected between the anode and cathode which is tuned to approximately the natural frequency of the piezoelectric element, a uni-directional current path connecting said cathode to said anode through sad tuned loop, a source of space current in said path, a potentiometer resistance connected to the terminals of said source, a direct current polarizing path connecting said screen grid to a point in said potentiometer which is positive with respect to the cathode, means electrically connecting the suppressor grid directly to the cathode, whereby extremely rapid changes in space current take place upon variation of the tuning of the closed loop circuit, and a control conductor electrically connected to the loop circuit to vary the tuning thereof upon change of the electrostatic environ ment of the control conductor.

10. In combination, a tuned circuit, a control conductor subject tocapacity variations connected to the tuned circuit, an electron discharge device comprising output electrodes connected to said tuned circuit and also comprising input electrodes, a piezoelectric element connected between said input electrodes and having a natural resonance frequency higher than that of the tuned circuit taken together with the control conductor with which the tuned circuit is connected, a grid leak path connected in shunt to the piezoelectric element, a space current path connected to the output electrodes of the discharge device and a source of space current therein, and means also included in the path actuated in response to large change in space current which occurs when the control conductor causes the natural resonance frequency of the tuned circuit to change.

11. In combination, a source of high frequency oscillations, said source comprising an electron discharge device having a cathode, an anode and an impedance control element, a piezoelectric element and means connecting said piezoelectric element to said cathode and impedance control element, a resonant circuit connected between said cathode and anode normally tuned to a frequency slightly lower than the natural frequency of said piezoelectric element whereby the oscillator normally oscillates at a point in the frequency zone in which the space current changes most rapidly with change of frequency of the resonant circuit, a capacity member connected to the resonant circuit to cause the frequency of the resonant circuit to vary in consequence of changes in the environment of the capacity member, and .a space current path between the cathode and anode including means responsive to large change in space current which occurs in consequence of changes in the environment of the capacity member.

12. In combination, an electric discharge device of the pentode type comprising a cathode, a control grid, a suppressor grid, an anode and a shield grid between the anode and the control grid, means for fixing the normal bias potentials of the suppressor grid and of the shield grid relative to the cathode, a circuit connected be tween the control grid and cathode, a second circuit connected between the anode and cathode, a piezoelectric element and a closed tuned loop both connected to the electric discharge device, the closed loop being tuned to a frequency slightly lower than the natural resonance frequency of the piezoelectric element, a high resistance grid leak path connected between the cathode and the control grid, a control conductor connected to the tuned loop and subject to variations in capacity to earth upon change of its electrostatic environment whereby the tuning frequency of the tuned loop correspondingly changes, a space current path including a source of space current connected between the cathode and anode, and means in the space current path actuated in response to the considerable change in the space current which occurs consequence of marked changes in the electrostatic environment of the control conductor.

CLIFFORD L. LOUDON. 

